OptionalconfigureConfigure the attached slot before its child starts loading.
OptionalconfigureConfigure each loaded object immediately before it replaces the slot.
OptionaldeferredChildren (by index) NOT to load now. Each keeps its slot, attached in authored order and empty, for the caller to load into later — it is never flattened and never removed.
OptionalorderStart order (indices into the children); default authored order.
Hooks for decorating temporary slots and their flattened children.